Over 15 Terawatts of heat is lost to the environment annually around the world by heat engines that require petroleum as their primary fuel source. This is because these engines only convert about 30 to 40% of petroleum's chemical energy into useful work. Waste heat generation is an unavoidable consequence of the second law of thermodynamics.
The term “thermoelectric effect” encompasses the Seebeck effect, Peltier effect and Thomson effect. Solid-state cooling and power generation based on thermoelectric effects typically employ the Seebeck effect or Peltier effect for power generation and heat pumping. The utility of such conventional thermoelectric devices is, however, typically limited by their low coefficient-of-performance (COP) (for refrigeration applications) or low efficiency (for power generation applications).
Thermoelectric device performance may be captured by a so-called thermoelectric figure-of-merit, Z=S2σ/k, where ‘S’ is the Seebeck coefficient, ‘a’ is the electrical conductivity, and ‘k’ is thermal conductivity. Z is typically employed as the indicator of the COP and the efficiency of thermoelectric devices—that is, COP scales with Z. A dimensionless figure-of-merit, ZT, may be employed to quantify thermoelectric device performance, where ‘T’ can be an average temperature of the hot and the cold sides of the device.
Applications of conventional semiconductor thermoelectric coolers are rather limited, as a result of a low figure-of-merit, despite many advantages that they provide over other refrigeration technologies. In cooling, low efficiency of thermoelectric devices made from conventional thermoelectric materials with small figure-of-merit limits their applications in providing efficient thermoelectric cooling.